The present invention relates to a method for detecting a focussing condition of an objective lens with respect to an object on which a light spot has to be focussed by said objective lens and to an apparatus for carrying out such a focus detecting method.
Such focus detecting method and apparatus are advantageously applied to an apparatus in which a scanning light spot is projected by an objective lens onto one or more information tracks recorded spirally or concentrically on a disc-shaped record medium to read an information recorded along the track.
In an embodiment of the apparatus for reproducing or picking-up an information from the above mentioned record medium, the record medium is called as a video disc in which encoded video and audio signals are recorded as optical information utilizing optical transmitting, reflection and phase properties. While the video disc is rotated at a high speed such as thirty revolutions per second, i.e. 1,800 rpm, a laser beam emitted from a laser light source such as a helium-neon gas laser is focussed on the track of the disc as a light spot and the optical information is read out therefrom. One of important properties of such a record medium is a very high density of recorded information and thus a width of the information track is very narrow and a space between successive tracks is also very narrow. In a typical video disc, a pitch of the tracks amounts only to 2 .mu.m. Therefore, the diameter of light spot should be correspondingly small such as 1 to 2 .mu.m. In order to pick-up correctly the recorded information from such tracks having very narrow width and pitch, an error in a distance between the objective lens and the tracks, i.e. a focussing error, should be reduced as little as possible to make a spot diameter as small as possible.
To this end, the apparatus is provided with a focussing control system in which an amount and a direction of a de-focussed condition of the objective lens with respect to the disc surface are detected to produce a focussing error signal and the objective lens is moved in a direction of the optical axis of objective lens in accordance with the detected focussing error signal.
FIG. 1 is a schematic view illustrating a known focus detection system in an optical pick-up apparatus. A light source 1 is constituted by a laser and emits light which is linearly polarized in a plane of the drawing of FIG. 1. The light is collimated by a collimator lens 2 into a parallel light beam which is then transmitted through a polarizing prism 3 and a quarter-wavelength plate 4. The light beam is further focussed by an objective lens 5 as a light spot on a disc 6 having one or more information tracks of crenellated pit construction. Then, the light is reflected by the information track and impinges upon the polarizing prism 3 by means of the objective lens 5 and the quarter-wavelength plate 4. The light impinging on the prism 3 is polarized in a direction perpendicular to the plane of the drawing, because it has transmitted through the quarter-wavelength plate 4 twice and thus, is now reflected by the polarizing prism 3. The light flux reflected by the polarizing prism 3 is converged by a condenser lens 7 and a cylindrical lens 8. Since the cylindrical lens 8 has a focussing power only in one direction, the shape of the focussed beam formed by the condenser lens 7 and the cylindrical lens 8 varies as shown in FIG. 1 with respect to an in-focussed condition in mutually orthogonal directions, when the disc 6 moves up and down. In the known apparatus, this variation in shape is detected by a light detector (not shown) divided into four sections and arranged at a focal plane of the lens system 7, 8 to produce a focussing error signal. The focussing error signal thus detected is supplied to a focussing mechanism such as a moving coil mechanism to move the objective lens 5 in its axial direction.
In the known focus detecting system, since a relatively long optical path is required to focus the light beam after being reflected by the polarizing prism 3, there is a drawback that an optical system is liable to be large in size. Further, since the light detector having the four sections must be arranged precisely in three axial directions, i.e. in the optical axis direction and in two orthogonal directions perpendicular to the optical axis, the adjustment in positioning the light detector is quite critical and requires a time-consuming work. Moreover, since a dynamic range in which the accurate focussing error signal can be obtained due to the deformation of the focussed beam is relatively small, any focussing error signal could not be produced if the disc deviates from a given position only by a relatively small distance.
The applicant has proposed a method which can obviate the above mentioned drawbacks and can detect a focussing error signal of an objective lens with respect to an object onto which a light spot is to be focussed, which method has an extremely high sensitivity for focus detection.
According to this method, in order to detect a focussing error signal of an objective lens with respect to an object on which a light spot is to be formed by means of said object lens, the following steps are carried out; focussing light emitted from a light source onto the object; introducing at least a part of a light flux reflected from the object into an optical member including an optical surface which reflects and/or refracts said part of light flux, said optical member being made of material which has a higher refractive index than that of material into which said light flux enters after being refracted by and transmitted through said optical surface; and detecting a variation in distribution of light amount of at least a part of light flux reflected and/or refracted by said optical surface to produce the focussing error signal.
FIG. 2 is a schematic view illustrating an optical pick-up apparatus for effecting the above mentioned focus detection method proposed by the applicant. In this method, an optical system for projecting a scanning light spot onto a record medium is same as that shown in FIG. 1. A linearly polarized light beam emitted from a laser light source 1 is collimated into a parallel light beam by a collimator lens 2 and passes through a polarizing prism 3 and a quarter-wavelength plate 4. Then, the parallel light beam impinges upon an objective lens 5 and is focussed on an information track of a disc 6 as a small light spot. The light beam reflected by the disc 6 is optically modulated in accordance with information recorded in the track and is reflected by the polarizing prism 3. The construction and operation of the optical system so far explained are entirely same as those of the known optical system shown in FIG. 1. The light flux reflected by the polarization prism 3 impinges upon a detection prism 10 having a reflection surface 11 and the light flux reflected by this surface 11 is received by a light detector 12. The reflection surface 11 is so arranged with respect to the incident light that under an in-focussed condition it makes a given angle with respect to the incident light (parallel light flux) which angle is equal to a critical angle or slightly smaller or greater than the critical angle. For a more detailed discussion concerning the reflection and refraction of light, reference is had to Fundementals of Physics, by Halliday and Resnick, published by John Wiley and Sons Inc. (chapter 36). Now, for the time being, it is assumed that the reflection surface 11 is set at the critical angle. In the in-focussed condition, the whole light flux reflected by the polarizing prism 3 is totally reflected by the reflection surface 11. In practice, a small amount of light is transmitted into a direction n shown in FIG. 2 due to incompleteness of a surface condition of the reflection surface 11. However, such a small amount of transmitted light may be ignored. If the disc 6 deviates from the in-focussed condition in a direction a in FIG. 2 and a distance between the objective lens 5 and the disc 6 is shortened, the light reflected by the polarizing prism 3 is no longer the parallel beam, but changes into a diverging light beam including extreme light rays ai.sub.1 and ai.sub.2. On the contrary, if the disc 6 deviates in the opposite direction b, the parallel light beam is changed into a converging light beam including extreme light rays bi.sub.1 and bi.sub.2. As can be seen in FIG. 2, light rays from an incident optical axis OP.sub.i to the extreme light ray a.sub.i1 have incident angles smaller than the critical angle and thus, are transmitted through the reflection surface 11 at least partially. Contrary to this, light rays between the optical axis OP.sub.i and the extreme light ray ai.sub.2 have incident angles larger than the critical angle and thus are totally reflected by the surface 11. In case of deviation of the disc 6 in the direction b, the above relation becomes inversed, and light rays below a plane which includes the incident optical axis OP.sub.i and is perpendicular to the plane of the drawing of FIG. 2, i.e. a plane of incidence, are totally reflected by the reflection surface 11, and light rays above said plane are at least partially transmitted through the reflection surface 11. As explained above, if the disc 6 deviates from the in-focussed position, the incident angles of the light rays impinging upon the reflection surface 11 vary in a continuous manner about the critical angle except for the center light ray passing along the optical axis OP.sub.i. Therefore, when the disc 6 deviates from the in-focussed position either in the direction a or b, the intensity of the light reflected by the reflection surface 11 varies abruptly near the critical angle in accordance with the above mentioned variation in the incident angles. In this case, senses of the variations of the light intensities on both sides of said plane perpendicular to the incident plane and including the incident optical axis OP.sub.i vary in mutually opposite manner. On the contrary, in the in-focussed condition, the light flux impinging upon the detection prism 10 is totally reflected by the reflection surface 11 and thus, the uniform light flux impinges upon the light detector 12. The light detector 12 is so constructed that the lower and upper light fluxes with respect to said plane are separately received by separate regions 12A and 12B, respectively. That is to say, the light detector 12 is divided along a plane which is perpendicular to the incident plane and includes an optical axis OP.sub.r of reflected light.
FIG. 3 shows a graph representing a variation of an intensity of reflected light in accordance with an incident angle near the critical angle. Curves R.sub.p and R.sub.s indicate the light intensities for P and S polarized light rays, respectively. The curves are obtained when the detection prism 10 is made of material having a refractive index of 1.50. It should be noted that an intensity of a non-polarized light ray is equal to an intermediate value of R.sub.p +R.sub.s /2.
In FIG. 2, if the disc 6 deviates in the direction a, the light rays of the lower half of the incident light flux have incident angles smaller than the critical angle. Therefore, at least a part of the lower half light flux is transmitted through the reflection surface 11 and the amount of light impinging upon the light receiving region 12A is decreased. While the upper half of the incident light flux has the incident angles larger than the critical angle and thus, is totally reflected by the surface 11. Therefore, the amount of light impinging upon the light receiving region 12B is not changed. On the contrary, if the disc 6 deviates in the direction b, the amount of light impinging upon the region 12B is decreased, but the amount of light impinging upon the region 12A is not changed. In this manner, the output signals from the regions 12A and 12B vary in an opposite manner. A focussing error signal can be obtained at an output 14 of a differential amplifier 13 as a difference signal of these signals from the regions 12A and 12B.
The reflection surface 11 may be set at an angle slightly smaller than the critical angle. In such a case when the disc 6 deviates in the direction a, the amount of light impinging upon the region 12B is first increased and then becomes constant and the amount of light impinging upon the region 12A is decreased abruptly. Whereas, if the disc 6 deviates in the direction b, the amount of light impinging upon the region 12A is first increased and then becomes constant, while the amount of light impinging upon the region 12B is decreased abruptly.
In this manner by detecting a difference in output signals from the light receiving regions 12A and 12B, it is possible to obtain the focussing error signal having an amplitude which is proportional to an amount of the deviation from the in-focussed condition and a polarity which represents a direction of the deviation with respect to the in-focussed condition. The focussing error signal thus obtained is used to effect a focussing control for driving the objective lens 5 in the direction of its optical axis. Further, it is possible to derive an information signal corresponding to the pit information recorded in the information track at an output 16 of an adder 15 which produces a sum signal of the output signals from the regions 12A and 12B. Further, in the in-focussed condition, since the light is scarcely transmitted through the reflection surface 11, a loss of light is very small and in the defocussed condition the half of light flux with respect to the central light ray is totally reflected, but an amount of the other half of light flux reflected by the surface 11 is decreased to a great extent, the difference in the amount of light impinging upon the regions 12A and 12B becomes great. Therefore, the very accurate focus detection can be effected with a very high sensitivity.
For instance, when use is made of the objective lens 5 having a numerical aperture NA=0.5 and a focal length f=3 mm and of the detection prism 10 having a refractive index n=1.50 and the disc 6 deviates by about 1 .mu.m, a variation of an incident angle for the extreme right ray which is subjected to the largest variation in incident angle is about 0.015.degree. which can cause a sufficiently large variation in light amount impinging upon the detector regions 12A and 12B.
In the apparatus shown in FIG. 2, the two light receiving regions 12A and 12B of the light detector 12 receive the light flux reflected by the reflection surface 11. It is also possible to arrange the two light receiving regions so as to receive the light flux transmitted through the reflection surface 11 or to arrange the two regions so to as receive the light fluxes reflected by and transmitted through the reflection surface, respectively.
The above mentioned focussing error signal detecting apparatus is based on the fact that the incident angle of light ray impinging upon the reflection surface varies when the disc deviates out of the in-focussed position. However, at a central region including the optical axis, the incident angle changes only slightly, whereas at a peripheral region the incident angle varies greatly. Therefore, the variation in intensity of the light impinging upon the light detector becomes smaller at the central region as compared with that at the peripheral region.
In the optically picking-up apparatus there is also produced a so-called tracking error due to a relative shift between the beam spot and the information track. When the tracking error occurs, the amount of light reflected by the disc 6 is also varied and this variation is detected by the light receiving regions. Therefore, noise might be introduced in the focussing error signal and thus, a signal-to-noise ratio of the focussing error signal might be decreased. Particularly, in the central region of the reflected light flux, information about the focussing error is inherently small and thus the influence of the tracking error upon the focus detection will be relatively enhanced. Therefore, the accuracy of the focussing error detection will be decreased to a great extent. Further, since the information recorded in the disc 6 is also reproduced by the light detector 12, the reproduced information signal might be subjected to noise due to the focussing error and a signal-to-noise ratio might be reduced owing to the focussing error.